1. Field of the Invention
This invention relates in general to integrated circuits, and in particular, to a method and apparatus for a high voltage switch circuit.
2. Description of Related Art
In many applications, MOSFET technology is being used to increase the speed, to reduce chip sizes, and to minimize power dissipation, etc. More and more applications require using MOSFET based technology. One concern related to implementing in most of today""s MOSFET based technology is that the MOSFET transistors are limited in terminal-to-terminal voltage to 5 volts or less. In addition, to achieve faster speeds and higher integration, the terminal-to-terminal voltages on the MOSFET transistors will continue to decrease. But, there continues to be circuit applications requiring high voltages in excess of, for example, 8 volts, that are designed using MOSFET technology, such as a high speed write driver in a computer disk drive.
A typical write driver is designed using bipolar NPN devices with greater than 5 volt breakdown voltages. But in order to turn on and off the write driver, a switch is needed which disconnects the write driver from a high voltage and forces the output terminals of the write driver to ground. In this disconnected state, no power must be dissipated by the write driver.
In previous and existing write drivers, a high voltage switch was designed using either a PNP or PFET device. In either case, the switching device needed to operate at voltages above 5 volts. However, with the faster MOSFET based technologies, the use of a PNP device that was once practical for this application is generally no longer available and the faster MOSFET devices are limited to less than 5 volt in their terminal-to-terminal voltage.
The previous and existing voltage sharing circuits, such as the one disclosed in U.S. Pat. No. 4,900,955, have described the use of low voltage FETs to obtain high voltage outputs by using a stack of FET devices connected in series with the drain electrode of one FET connected to the source electrode of the next FET in the stack. However, none of the previous and existing voltage sharing circuits provide or teach the switch functions in a high voltage application with MOSFET technology, such as in a write driver circuit of a disk drive. Furthermore, none of the previous and existing voltage sharing circuits disclose or teach the use of a bipolar transistor to control bias to diodes and stacked FETs.
Therefore, there is a need for a high voltage switch to provide a high voltage switch function in the write driver circuit of a disk drive which is designed with the latest MOSFET technology having low terminal-to-terminal voltages.
There is also a need to have a method and apparatus suitable for high voltage applications which are designed with the latest MOSFET technology having limited terminal-to-terminal voltage capability.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for a high voltage application, and in particular, to a method and apparatus for a high voltage application with MOSFET technology.
In general, the present invention solves the above-described problems by including at least two serially coupled MOSFETs and a two-stage MOSFET connection using a plurality of resistors, and/or a bipolar transistor, and a plurality of diodes.
One of the high voltage applications of the present invention is the writing of data onto a disk in a disk drive. In one embodiment, a high voltage write driver in the Arm Electronics (AE) chip requires a high voltage switch to turn on and off the high voltage to the write driver. The technology used in the AE chip has a maximum terminal-to-terminal voltage of the MOSFET transistors of 5 volts. However, the switch circuitry is required to switch a voltage in excess of, for example, 8 or 9 volts. When the switch is on, the output of the switch applies the high voltage to the write driver. When off, the switch output switches to ground forcing the output of the write driver to the ground which turns off all power in both the write driver and the voltage switch.
A further aspect of the one embodiment of the present invention is a high voltage switch using a technology that has a maximum terminal-to-terminal voltage for MOSFET transistors of less than 5 volts, such as 3.3 or 2.5 volts, etc., yet still requiring voltage levels on the switch that are greater than the terminal-to-terminal voltage of the MOSFETs.
The present invention provides a switch circuitry capable of providing an output voltage between ground and the High Voltage (HV) power supply which is higher than the maximum terminal-to-terminal voltage of the MOSFET transistor. In one embodiment, the switch circuitry includes at least a first PFET and a second PFET. The first PFET has a source coupled to the HV power supply and a drain coupled to a source of the second PFET. The second PFET has a drain coupled to a first resistor where the other end of the first resistor coupled to the ground. The drain of the second PFET is the output of the switch. When the switch is on, the voltage on the gates of both PFETs is dropped to a level to turn on the PFETs but not to violate the maximum terminal-to-terminal voltage of the PFET. In the case of this embodiment, the voltage on the gates of the PFETs is dropped to 5 volts below the HV power supply. This turns on both PFETs providing a low resistance path from the HV power supply to the output of the switch. To turn the switch off, the voltage on the gate of the second PFET is first raised to a level which is just under the maximum terminal-to-terminal voltage of the PFET. In the case of this embodiment, the voltage on the gate of the second PFET is raised to 5 volts. Once the gate on the second PFET is at 5 volts, the voltage on the gate of the first PFET is then raised to the HV power supply turning both PFETs completely off. With both PFETs off, the output of the switch drops to ground through the resistor coupled to ground. To turn on the high voltage switch, the voltage on the gate of the second PFET is first dropped to 5 volts below the HV power supply and then the voltage on the gate of the first PFET is dropped to 5 volts below the HV power supply, turning on both PFETs. This embodiment uses two PFETs to accomplish the high voltage switch without violating the maximum terminal-to-terminal voltage of the PFETs. It will be appreciated that if the maximum terminal-to-terminal voltage is considerably less than 5 volts, additionally PFETs can be added to implement the high voltage switch according to the principles of the present invention.
One aspect of the present invention is that the first power supply is nine volts, the second power supply is five volts, and the ground is zero volt.
Another aspect of the present invention is that the first, second, and third diodes are Schottky diodes. Other diodes with a higher reverse breakdown voltage can be used for the first, second, and third diodes.
A further aspect of the present invention is that the output of the switch circuitry is zero when the switch circuitry is switched off. The input is approximately zero volt when the switch circuitry is switched off.
Additional aspect of the present invention is that the first, second, third, fourth, and fifth resistors can be modified for different voltages of the second power supply (Vccd) and the first power supply (HV).
Still another aspect of the present invention is that the output voltage of the switch circuitry is the same as the first power supply (HV) when the switch circuitry is switched on.
Further another aspect of the present invention is that the bipolar transistor is a NPN transistor.
The present invention is also a disk drive apparatus capable of writing data to a plurality of disks, including: a plurality of write heads capable of writing data to the plurality of disks, respectively; a plurality of write drivers, each of the plurality of write drivers coupled to each of the plurality of write heads, respectively, and enabling/disabling the writing of data to the respective disk; a plurality of switch circuits, each of the plurality of switch circuits coupled to each of the plurality of write drivers, respectively, and being capable of switching on/off at least one of the plurality of write drivers; a controller coupled to the switch circuits and controlling switching on/off each of the switch circuits; and wherein when the controller selects to switch on one of the switch circuits, the respective write driver writes data on the respective disk, an input of the selected switch circuit is approximately a predetermined voltage (Vccd), and an output of the selected switch circuit is higher than the predetermined voltage, e.g. HV.
One aspect of the disk drive apparatus invention is that the predetermined voltage is five volts, and the output of the selected switch circuit is nine volts.
Another aspect of the present invention is that when the controller switches off the selected switch circuit, the input of the selected switch circuit is zero volt, and the output of the selected switch circuit is zero volt.
Still another aspect of the present invention is that the predetermined voltage ranges from 2.8 volts to 5 volts.
Further, the present invention is also a method of writing data in a disk drive apparatus having a plurality of write drivers coupled to a plurality of write heads for writing data to a plurality of disks, each of the plurality of write drivers coupled to a switch circuit, including the steps of: selecting a write driver for writing on a track position of one of the disks; sending a write signal to the respective switch circuit of the selected write driver by a controller; switching on the respective switch circuit of the selected write driver which outputs to the respectively coupled write head a voltage higher than a predetermined input voltage of the respective switch circuit to enable the respective write driver to write data on the respective disk; and writing data on the respective disk via the respective write head.
The present invention also provides a high voltage switch circuit capable of providing an output voltage between ground and a power supply voltage which is higher than a predetermined voltage, including: a first FET device; a second FET device coupled to the first FET device in series between the ground and the first power supply voltage, the second FET having a drain being an output of the switch circuit; a first diode coupled to a gate of the first FET device; a second diode coupled between a source and a gate of the second FET device; a third diode coupled between the first diode and the second diode; a bipolar transistor having a collector coupled to both the first and third diodes; a third FET device coupled between an emitter of the bipolar transistor and the ground, the third FET device having a gate being an input of the switch circuit; and wherein the bipolar transistor controls voltage bias to the diodes and the first and second FET devices.
Other embodiments of an apparatus and method in accordance with the principles of the invention may include alternative or optional additional aspects. For example, the switch circuitry can be configured and arranged to use the NMOS (NFET) instead of the PMOS (PFET). It will be appreciated that other functionally equivalent alternative embodiments of the switch circuitry can be used without departure of the present invention.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.